The invention relates to capacitive pick-off circuits; that is, circuits that are used to determine the capacitance or the gap between plates of a capacitor.
In prior art capacitive pick-off systems, such as described in the copending application of Holdren et al Ser. No. 702,389, filed on July 6, 1976, assigned to the assignee of this application, capacitance is measured by applying a time varying voltage such as a triangular or saw-tooth waveform to the capacitor plates. The resulting current through the capacitor is then used as a measure of the capacitance or the gap between the plates of the capacitor. However, due to the fact that the capacitance of the capacitor varies in an inverse relationship with the distance or gap between the plates, a change in this gap will result in nonlinearities in the output current which can, under some circumstances and in certain applications of capacitive pick-off circuits, lead to significant errors. For example, a ten percent change in the gap will give rise to over a one percent nonlinearity, a twenty percent gap change will give rise to over a four percent nonlinearity and a fifty percent gap change will result in a thirty percent nonlinearity. Also, since the output current of the capacitor increases substantially as the plate gap becomes very small, significant stability problems can appear when the capacitor is used as a position sensing element in a servoed device. Capacitive pick-off circuits are used in a large number instrumentation type applications such as transducers and accelerometers and for some highly sensitive instruments such as servoed accelerometers and transducers where nonlinearities due to significant changes in the capacitor plate gap can be a significant source of error.